Add in-queue check for cancelTransaction() in Timelock.sol

[AmazingAng]

Add in-queue check for cancelTransaction() in Timelock.sol :

        require(queuedTransactions[txHash], "Timelock::cancelTransaction: Transaction hasn't been queued.");

before deleting the transaction from the queue:

        queuedTransactions[txHash] = false;

Link to #214

[AmazingAng]

1. Delete import SafeMath, since SafeMath is no longer needed starting with Solidity 0.8.
2. Delete keyword "public" in constructor